The Pacific off the west coast can be rough during the months of heavy swells (Nov 15th to March 1st) Strong Storms in the Pacific Northwest create the swells as they move from west to east across the globe. Fortunately (unfortunately if you are a surfer) they only happen a very few times per year with enough intensity to make the ride rough.
When ships return through these swells from Mexico they buck them all the way home as there is no way to avoid them as they come head on.
I wouldn't worry about it however, I have never had rough seas returning from Mexico and I can't tell you how many times I have cruised there. Both ships are excellent choices and I really liked the Mercury's deep Mexican Riviera itinerary that included Acapulco, Ixtapa/Zihuatanejo, Manzanillo, Puerto Vallarta, Mazatlan and Cabo San Lucas.
Have a great cruise!